Method for reduction treatment of metal oxide or ironmaking waste and method for concentration and recovery zinc and/or lead

ABSTRACT

A method of reduction treatment of metal oxides characterized by using as a material a powder containing metal oxides and containing alkali metals and halogen elements and further, in accordance with need, carbon, mixing said material with water to produce a slurry, then dehydrating this and charging the dehydrated material, mixed with another material in accordance with need, into a rotary hearth type reduction furnace for reduction.

TECHNICAL FIELD

The present invention relates to technology for reducing metal oxides ina rotary hearth type reduction furnace using a powder including powdermetal oxides as a feed material and for removing alkali metals, halogenelements, and other impurities.

Further, it relates to technology for separating and recovering zinc andlead from dust of a refining furnace by a rotary hearth type reductionfurnace.

Further, the present invention relates to a method of treatment andsystem of treatment of steelmaking waste which reduce by heating dust,sludge, and other steelmaking waste containing iron oxide produced inthe steel refining and processing processes so as to mainly recover zincoxide (ZnO).

BACKGROUND ART

There are various reduction processes for producing reduced iron oralloy iron. Among these, there is the process using powder metal oxidesas feed materials to produce pellets and reducing these pellets at ahigh temperature.

This type of process is performed using a reduction furnace. As examplesof such a reduction furnace, there are a shaft type hydrogen gasreduction furnace, rotary kiln type reduction furnace, rotary hearthtype reduction furnace, etc.

Among these, a shaft type hydrogen gas reduction furnace reduces pelletsmade from powder ore by hydrogen gas.

On the other hand, in a rotary kiln type reduction furnace and rotaryhearth type reduction furnace, heat is supplied to the reduction furnaceand a reduction reaction is caused by the carbon mixed into the pellets.That is, in a rotary kiln type reduction furnace and rotary hearth typereduction furnace, shaped articles. (pellets) obtained by mixing andshaping coal, coke, or other carbon and powder metal oxides are reduced.

The method of producing reduced iron using a rotary kiln type reductionfurnace or rotary hearth type reduction furnace can use inexpensive coaletc., so is drawing attention as an economical method of producingreduced iron.

A rotary hearth type reduction furnace is a furnace of a type where adisk-shaped refractory hearth with a center cut away is rotated on railsat a constant speed under a fixed refractory ceiling and side walls. Thediameter of the rotating hearth is 10 to 15 meters, while the width ofthe hearth is 2 to 6 meters.

While the hearth is rotating, the feed material feeder, heat zone,reduction zone, and product ejector are successively moved. The shapedarticles of the feed material are charged from the feed material feeder.After this, the shaped articles are heated in the heat-zone to about1200° C. or more, then the carbon and metal oxides react in thereduction zone whereby a metal is produced.

In a method of reduction using a rotary hearth, the heating is quicklyperformed by radiation, so the reduction reaction ends in 7 to 20minutes. The reduced shaped articles are ejected from the furnace andcooled, then used as a feed material for an electric furnace or blastfurnace.

In this way, in a rotary hearth type reduction furnace, powder mainlycomprised of carbon and metal oxides is used as the shaped articles forreduction by heating. In general, powders of at least three types offeed materials are used. This is to adjust the ratio between the metaloxides and carbon and adjust the composition of the particles sizes whenproducing the shaped articles.

In operation, these feed materials are mixed to produce the shapedarticles. At this time, the feed material powders are mixed in apredetermined ratio so as to ensure a suitable chemical composition andparticle size composition. The result is shaped by a molding machine.

In the method of reduction using a rotary hearth type reduction furnace,ore is generally used as the powder containing the metal oxides, butsometimes the dust or sludge produced in the metal refining process orprocessing process is used.

In particular, the dust or sludge produced in the ferrous metal industryincludes zinc, lead, and other impurities. These evaporate at areduction temperature of 1200° C. or more. Therefore, a rotary hearthtype reduction furnace is an effective means for removing impurities.

In this way, in a rotary hearth type reduction furnace, the zinc, lead,and other impurities mixed in the shaped articles (pellets) form dustcomponents in the exhaust gas. When the concentration of zinc or lead inthe dust is high, the dust is used as a zinc feed material or lead feedmaterial in nonferrous metal refineries.

To enable stable operation of a rotary hearth type reduction furnace, itis important to suitably adjust the chemical composition of the shapedarticles (pellets). In regard to the operation, at the time of reducingthe Zost general iron oxide, the amounts of the iron oxide and carbon,mainly the ratio of the iron oxide and carbon, are important asingredients to be adjusted.

When using a powder feed material including, in addition to zinc andlead, alkali metals and halogen elements mixed in, special considerationis required for volatile substances mixed into the iron oxide inaddition to adjustment of the ratio of the iron oxide and carbon. Theinventors, as disclosed in Japanese Unexamined Patent Publication(Kokai) No. 2003-090686 (Japanese Patent Application No. 2001-279055 ofthe same inventors), found that if the exhaust gas of a rotary hearthtype reduction furnace contains a large amount of sodium chloride,potassium chloride, or other alkali metal halides, due to the (i)problem that these substances deposit as dust inside the exhaust gastreatment system and become factors inhibiting operation and the (ii)problem that the concentration of zinc in the dust falls and the valueas a zinc feed material falls, it is important to prepare the feedmaterial under the following conditions.

That is, as disclosed in Japanese Unexamined Patent Publication (Kokai)No. 2003-090686, the total number of moles (A) of zinc and lead in thefeed material, the total number of moles (B) of potassium and sodium,and the total number or moles (C) of chlorine and fluorine are made tosatisfy the relationship of (0.8C-0.7B)/A<0.36. By preparing the feedmaterial to satisfy these conditions, the deposition of dust inside theexhaust gas treatment system can be suppressed and stable operation overa long period of time becomes possible.

Further, as shown in Japanese Unexamined Patent Publication (Kokai) No.2000-169906, the inventors have proposed to improve the structure of theexhaust gas treatment system and control the exhaust gas temperature soas to suppress deposition of dust. By combining these measures againstdust deposition and the restrictions in feed material ingredients in theinvention described in Japanese Unexamined Patent Publication (Kokai)No. 2003-090686, it is possible to maintain a stable operating state ofthe exhaust gas treatment system.

Accordingly, in the prior art, the feed material for use in a rotaryhearth type reduction furnace was analyzed for composition in advanceand the conditions on the feed material such as the contents of zinc andlead and halogen elements and alkali metals were restricted and theexhaust gas treatment system modified so as to solve the above problems.

However, the method of ensuring stable operation while preventing dustdeposition in an exhaust gas-treatment system of a rotary hearth typereduction furnace by restricting the contents of the zinc and lead andthe halogen elements and alkali metals in the feed material suffers fromthe problem that the feed material is restricted to feed materialcontaining relatively little amounts of halogen atoms and alkali metals.

For example, when treating dust containing iron oxide or pickling sludgeof steel products produced at steelmaking plants, the content of thepotassium chloride or sodium chloride in these feed materials is highand application of the method described in Japanese Unexamined PatentPublication (Kokai) No. 2003-090686 is difficult.

For example, the dust contained in blast furnace gas often contains atotal of 1 mass % of potassium chloride or sodium chloride. Further, thesludge produced by pickling of steel products often contains thehydrochloric acid or fluoric acid used at the time of pickling andresidual matter. In both cases, there is the problem that the above feedmaterial conditions cannot be satisfied.

As a result, when treating dust or sludge by a rotary hearth typereduction furnace, the problem has arisen of the dust depositing at thegas passages of the boiler or heat exchanger for reclaiming waste heatand, other locations inside the waste gas treatment system. That is, thezinc oxide or lead oxide end up containing a certain percentage or moreof alkali metals and halogen elements.

Some of the dust generated from a rotary hearth type reduction furnace(hereinafter referred to as “secondary dust”) contains alkali metals andhalogen elements in a high rate of 20 to 45 mass %. As a result of thishigh rate of content, an inorganic mixture containing zinc oxide, zincchloride, sodium chloride, potassium chloride, etc. mixed together isformed. This substance has a low melting point of 600° C. or less.

The secondary dust containing alkali metals and halogen elements-in highconcentrations exhibits an extremely high deposition ability underconditions of 400 to 600° C. This deposits on the gas passages of theboiler or heat exchanger to clog the exhaust gas channels and obstructoperation of the rotary hearth type reduction furnace.

In this way, in the case of a feed material containing large amounts ofalkali metals and halogen elements, with just the prior art, theseelements had a detrimental effect and stable operation was not possible.

Note that this dust deposition ability becomes higher when the ratio ofthe sodium chloride, potassium chloride, etc. with respect to the zincoxide (partially zinc chloride) is high. Further, it becomes higher whenthe ratio of the sodium chloride, potassium chloride, etc. themselves ishigh.

Further, in a rotary hearth type reduction furnace, when performingrecycling treatment consisting of introducing a carbon-bearing materialor other reductant to the dust, sludge, or other steelmaking wastecomprised mostly of iron oxide, but also containing a large amount ofzinc and heating the result to reduce, evaporate away, and reoxidize thezinc oxide (ZnO) contained in the steelmaking waste so as to therebyrecover it at a dust collector as secondary dust, the secondary dustwill contain zinc in a high concentration, so the secondary dust can beutilized as a zinc feed material.

If the concentration of zinc in the zinc-containing secondary dust(converted to metallic zinc, hereinafter referred to as T. Zn, the samefor the concentration of lead, which is converted to metallic lead andreferred to as T. Pb) is 50 to 55%, the quality becomes one able to bedirectly utilized for a zinc blast furnace. This secondary dust isvaluable as a zinc feed material.

However, when using such a feed material containing large amounts ofalkali metals and halogen elements, the sodium chloride, potassiumchloride; etc. of the feed material migrate to the secondary dust andcause the problem of a drop in the zinc concentration of the secondarydust.

In some cases, the concentration of alkali metals and halogen elementsbecomes 30 mass % or more and the T. Zn becomes a low 30 to 40 mass %,whereupon the secondary dust can no longer be directly used in a zincblast furnace.

In particular, halogen elements inhibit the reaction at the time of zincrefining, so restriction of their quantity is an important item inmanagement in zinc refining.

That is, to recycle recovered secondary dust as a zinc feed material, itis necessary to remove harmful substances from the low zincconcentration, high halogen concentration secondary dust bypre-treatment to concentrate the zinc further, this pre-treatmentrequires massive costs, so the cut-cutting and energy-saving effects tobe inherently enjoyed due to recovery of zinc oxide are reduced and, inthe worst case, cancelled out entirely.

Therefore, conventionally, only steelmaking waste with little chlorinecontent is selected as a feed material. Steelmaking waste with a highchlorine content has been deemed to have no merits for treatment andtherefore has not been used as a recycling material.

Further, substantially the same problems occurred as with zinc whenrecycling lead as well.

As technology for recovering zinc, lead, and other valuable metals fromdust including iron oxide, for example, Japanese Examined PatentPublication (Kokoku) No. 53-29122 discloses technology comprised of astep of washing the dust etc. to remove the chlorine, sodium, andpotassium, a step of adding coke to the washed dust obtained at thisstep and granulating and sintering the same to obtain sintered iron orecontaining zinc and lead, and a step of washing the sintered dustobtained by removing the dust from the sintering gas from this step byalkali water to remove the fluorine to obtain nonferrous metal slagcontaining lead and cadmium.

However, this technology, as described in Japanese Examined PatentPublication (Kokoku) No. 53-29122 (see page A, column 6), does notreduce or vaporize the zinc in the sintering step, but leaves it in thesintered ore, so it is necessary to separately prepare a standing typedistillation furnace etc. to reduce, vaporize, and recover the zinc.

Further, in this technology, the dust washed by the water contains 30 to40% moisture, so as described in this publication (see page 2, column4), it is necessary to dry it in a rotary drier etc. before sintering.

PCT Publication Pamphlet Wo 01/42516 A1 discloses mixing by agitation apowder containing metal oxides and, carbon in a state containing atleast 1.0 times the moisture with respect to the powder weight,dehydrating this by a dehydration system until a moisture content of 16to 26 mass %, then shaping it by a compression molding machine toproduce shaped articles of a powder filling rate of 0.43 to 0.58,charging the shaped articles into a rotary hearth type reduction furnacehaving an atmospheric temperature of 1170° C. or less, and reducing themby sintering at 1200° C. or more.

Further, Japanese Unexamined Patent Publication (Kokai) No. 2001-303115discloses technology of dehydrating a slurry of a mixture of powdercontaining metal oxides and powder containing carbon by a double-rollpress type dehydrator down to a moisture content of 16 to 27%, producingshaped articles by an extrusion molding machine, and reducing bysintering the articles by a rotary hearth type reduction furnace toobtain metal.

However, in this technology, the high moisture content powder is chargedinto the reduction furnace without any drying step, so removal of thesodium chloride, potassium chloride, and other volatile harmfulsubstances is not considered.

In this way, when using a rotary hearth type reduction furnace to treata feed material containing large amounts of alkali metals and halogenelements, the following problems occur. In particular, there wereproblems in stable operation of the rotary hearth type reduction furnaceand the pre-treatment for converting the secondary dust to a goodquality zinc feed material.

Therefore, new technology enabling stable operation and pre-treatment ofsecondary dust economically even when using a feed material containinglarge amounts of alkali metals and halogen elements has been sought.

SUMMARY OF THE INVENTION

The present invention solves the above problem points in the prior artby providing a reduction treatment method and reduction treatment systemcomprising reducing by heating a powder containing powder metal oxidesor dust or sludge or other steelmaking waste containing iron oxideproduced in the steel refining and processing processes by a rotaryhearth type reduction furnace or a moving hearth type reduction furnaceso as to reduce the metal oxides and separately recovering the volatileharmful substances (alkali metals, halogen elements, and otherimpurities) and zinc oxide (ZnO) and further a zinc and/or leadconcentration and recovery method.

(A) The gist of the invention for reduction treatment of powdercontaining powder metal oxides is as follows:

(1) A method of reduction treatment of metal oxides characterized byusing as a feed material a powder containing metal oxides and containingalkali metals and halogen elements, mixing said feed material with waterto produce a slurry, then dehydrating this and charging the dehydratedmaterial into a rotary hearth type reduction furnace for reduction.

(2) A method of reduction treatment of metal oxides characterized byusing as a feed material a powder containing metal oxides and containingalkali metals and halogen elements, mixing said feed material with waterto produce a slurry, then dehydrating this, mixing the dehydratedmaterial with another feed material, and charging said mixture into arotary hearth type reduction furnace for reduction.

(3) A method of reduction treatment of metal oxides characterized byusing as a feed material a mixed powder of a powder containing metaloxides and containing alkali metals and halogen elements and a powdercontaining carbon, mixing said feed material with water to produce aslurry, then dehydrating this, and charging said dehydrated materialinto a rotary hearth type reduction furnace for reduction.

(4) A method of reduction treatment of metal oxides characterized byusing as a feed material a mixed powder of a powder containing metaloxides and containing alkali metals and halogen elements and a powdercontaining carbon, mixing said feed material with water to produce aslurry, then dehydrating this, mixing the dehydrated material withanother feed material, and charging said mixture into a rotary hearthtype reduction furnace for reduction.

(5) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (4), characterized in that said powder containsa total of at least 0.1 mass % of alkali metals and halogen-elements.

(6) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (5), characterized in that a mass ratio ofpowder and water in said slurry is at least 1:1.5 and a mass-ratio ofpowder and water in said dehydrated material is not more than 1:0.4.

(7) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (6), characterized by heating and agitating theslurry at 80° C. or less in the production of said slurry.

(8) A method of reduction treatment of metal oxides as set forth in anyone of the above (1), (2), and (5) to (7), characterized by using assaid feed material a powder containing both iron oxide and zinc oxideand/or lead oxide and containing alkali metals and halogen elements in aratio alkali/(zinc+lead) between a total of the number of moles ofalkali salts and a total of the number of moles of lead of at least 0.1.

(9) A method of reduction treatment of metal oxides as set forth in anyone of the above (3), (4), and (5) to (7) characterized by using as saidfeed material a powder comprised of a mixture of a powder containingboth iron oxide and zinc oxide and/or lead oxide and a powder containingcarbon and containing alkali metals and halogen elements in a ratioalkali/(zinc+lead) between a total of the number of moles of alkalisalts and a total of the number of moles of lead of at least 0.1.

(10) A method of reduction treatment of metal oxides as set forth in theabove (6), characterized in that a pH of a slurry produced by mixingsaid powder with water is 7 to 11.5.

(11) A method of reduction treatment of metal oxides as set forth in theabove (9), characterized in that a pH of a slurry produced by mixingsaid mixed powder with water is 7 to 11.5.

(12) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (11), characterized by shaping said dehydratedmaterial into moist shaped articles having a porosity of at least 35%and charging said shaped articles into a rotary hearth type reductionfurnace for reduction without drying.

(13) A method of reduction treatment of metal oxides as set forth in theabove (12), characterized by making a mass ratio of powder and water insaid dehydrated material 1:0.2 to 1:0.4 and shaping said dehydratedmaterial into moist shaped articles having an average volume of not morethan 10000 mm³.

(14) A method of reduction treatment of metal oxides as set forth in theabove (13), characterized by making a molar ratio of oxygen and carboncontained in said shaped articles 1:0.6 to 1:1.5, charging said shapedarticles into a rotary hearth type reduction furnace, and reducing themby leaving them for at least 8 minutes at the part of the furnace havinga gas temperature or 1200° C. or more.

(15) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (14), characterized in that said rotary hearthtype reduction furnace is provided with an exhaust gas treatmentfacility having at least one of a waste heat boiler and an airpreheater.

(16) A method of reduction treatment of metal oxides as set forth in anyone of the above (1) to (15), characterized in that said powder issteelmaking waste.

(B) The gist of the invention for concentrating and recovering zincand/or lead is as follows:

(17) A method of concentrating and recovering zinc and/or leadcharacterized by recovering dust in exhaust gas produced in the methodof reduction treatment of metal oxides described in any of the above (1)to (16) as feed material for zinc and/or lead.

(C) The gist of the invention for reduction treatment of dust, sludge,or other steelmaking waste including iron oxide is as follows:

(18) A method of reduction treatment of steelmaking waste characterizedby:

mixing by agitation steelmaking waste, a pH adjuster, and acarbon-bearing material in water, then concentrating the mixture toproduce a slurry,

pressing said slurry to dehydrate it,

extruding said dehydrated material to shape it into shaped articles,

charging said shaped articles into a moving hearth type reductionfurnace for reduction and recovering the secondary dust containing zincoxide produced.

(19) A method of reduction treatment of steelmaking waste characterizedby:

stirring and mixing steelmaking waste and a pH adjuster in water, thenconcentrating the mixture to produce a slurry,

pressing said slurry to dehydrate it,

adding and kneading a carbon-bearing material into said dehydratedmaterial,

extruding said dehydrated material to shape it into shaped articles,

charging said shaped articles into a moving hearth type reductionfurnace for reduction and recovering the secondary dust containing zincoxide produced.

(20) A method of reduction treatment of steelmaking waste as set forthin the above (18) or (19) characterized in that said pH adjuster is asubstance containing OH— groups.

(21) A method of reduction treatment of steelmaking waste as set forthin any one of the above (18) to (20) characterized in that said pHadjuster is fly ash discharged from a refuse melting furnace orincinerator furnace.

(22) A method of reduction treatment of steelmaking waste as set forthfin any one of the above (18) to (21) characterized in that a pH of theslurry adjusted in pH by said pH adjuster is at least 8.

(23) A method of reduction treatment of steelmaking waste as set forthin any one of the above (18) to (22) characterized in that saiddehydrated material contains moisture in an amount of 16 to 27 mass % ofsaid dehydrated material.

(24) A system for reduction treatment of steelmaking waste characterizedby being provided with:

an agitation tank for mixing by agitation steelmaking waste, a pHadjuster, and a carbon-bearing material in water,

a concentration tank for concentrating the agitated mixture to produce aslurry,

a dehydrator for pressing the slurry poured on endlessly moving filtercloth by at least one pair of rolls arranged above and below the clothso as to dehydrate it,

a molding machine for extruding said dehydrated material from a die toshape it,

a moving hearth type reduction furnace for reducing said shapedarticles, and

a dust collector for-recovering the secondary dust containing zinc oxideproduced in said moving hearth type reduction furnace.

(25) A system for reduction treatment of steelmaking waste characterizedby being provided with:

an agitation tank for mixing by agitation steelmaking waste and a pHadjuster in water,

a concentration tank for concentrating the agitated mixture to produce aslurry,

a dehydrator for pressing the slurry poured on endlessly moving filtercloth by at least one pair of rolls arranged above and below the clothso as to dehydrate it,

a kneader for adding and kneading a carbon-bearing material to saiddehydrated material,

a molding machine for extruding said dehydrated material from a dies toshape it,

a moving hearth type reduction furnace for reducing said shapedarticles, and

a dust collector for recovering the secondary dust containing zinc oxideproduced in said moving hearth type reduction furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of an embodiment of the present invention for reductiontreatment of steelmaking waste using a rotary hearth type reductionfurnace.

FIG. 2 is a view of another embodiment of the present invention forreduction treatment of steelmaking waste using a rotary hearth typereduction furnace.

FIG. 3 is a view of an embodiment of the present invention for reductiontreatment of steelmaking waste using a moving hearth type reductionfurnace.

FIG. 4 is a view of another embodiment of the present invention forreduction treatment of steelmaking waste using a moving hearth typereduction furnace.

THE MOST PREFERRED EMBODIMENT

(I) Regarding Invention for Reduction Treatment of Powder ContainingPowder Metal Oxides and Invention for Concentrating and Recovering Zincand/or Lead

An example of a facility for working the present invention is shown inFIG. 1. The facility of FIG. 1 is comprised of an agitation tank 1, aslurry pump 2, a dehydrator 3, a molding machine 4, a rotary hearth typereduction furnace 5, a reduced shaped article cooling device 6, anexhaust gas discharge duct 7, a waste heat boiler 8, a heat exchanger 9,a dust collector 10, and a flue 11.

The present invention dissolves all alkali metal halides (hereinafterreferred to as “alkali salts”) in water to remove them from the feedmaterial powder and reduces the feed material powder in the rotaryhearth type reduction furnace 5.

Note that here the example is taken of a feed material comprised of amixture of mainly alkali salts so as to explain the method of thepresent invention, but the present invention is a method effective alsofor the reduction treatment of feed materials containing water-solublecompounds of alkali metals and halogen elements.

Such water-soluble compounds of alkali metals and halogen elementsinclude sodium carbonate, potassium nitrate, sodium sulfate, ammoniumchloride, etc.

At least two types of the feed material powder, if possible three typesto six types, are prepared. Ones containing large amounts of metaloxides and ones containing carbon are prepared.

In general, fine powder ore, iron sand, fine powder coke, powdercharcoal, dust produced from steelmaking blast furnaces, dust producedfrom steelmaking converters, dust produced from steelmaking electricfurnaces, sludge produced in the process of pickling ferrous metalproducts or stainless steel products, etc. are used. It is also possibleto use the dust including iron oxide and zinc produced when treatingsteelmaking dust by a rotary kiln etc.

Further, according to the present invention, it is possible to treat bya rotary hearth type reduction furnace even the fly ash containing largeamounts of sodium chloride and zinc and lead produced in melting typerefuse incinerator furnaces—which could not be treated by a rotaryhearth type reduction furnace in the prior art.

First, the chemical compositions of these feed material powders areanalyzed. It is preferable to determine the ratios of blending of thedifferent feed material powders based on the results of analysis. Thematter of the greatest priority when adjusting the composition is toenable sufficient reduction of the oxides of the main target metal, sofirst the ratio of the reducible metal oxides and carbon is determined.

Here, the “reducible metal oxides” indicate oxides reduced by carbonmonoxide at about 1300° C. and include iron oxide, manganese oxide,nickel oxide, zinc oxide, lead oxide, etc.

The molar ratio of the oxygen (active oxygen) and carbon bonded withthese reducible metal oxides is adjusted to a suitable value. The molarratio of the active oxygen and carbon is made 1:0.6 to 1:1.5.

Note that this molar ratio-is the ratio between the value obtained bydividing the mass ratio of the content of the carbon by the atomicweight [12] of carbon and the value obtained by dividing the mass ratioof the content of the active carbon by the atomic weight [16] of oxygen.

At the time of the analysis of composition, it is preferable to obtainthe elemental analysis values of the sodium, potassium, chlorine,fluorine, zinc, and lead in the feed material powder. Further, ifnecessary, it is also possible to obtain the analysis values forbromine, lithium, etc. further, it is preferable to find the contents ofthe alkali salts by analysis.

However, quantitative analysis of alkali salts takes time, so it is alsopossible to estimate the contents of the alkali salts based on theanalysis values of the elements.

In general, the analysis values of the elemental mass ratios of sodium,potassium, chlorine, and fluorine are obtained and the contentscalculated from the balance of the anions and cations.

When the ratio of the chlorine, fluorine, and cations is greater thanthe chemical equivalents of the alkali salts, part of the chlorine orfluorine forms iron chloride, calcium chloride, zinc chloride, etc., sothe weight of the alkali salts calculated from the small amount ofcations and the molecular weight of the alkali salts is made theestimated content.

In this case, the excess chlorine and fluorine bond with elements otherthan the alkali metals by ratios proportional to the mass ratios.

Further, when the elemental analysis shows that the number of cations ofthe sodium or potassium is high, the mass of the alkali salts calculatedfrom cations and the molecular weights of the alkali salts is made theestimated content.

In this case, the excess sodium and potassium bond with elements otherthan the halogens by ratios proportional to the mass ratios.

Further, the contents of the zinc and lead are analyzed to find theratio of the total number of moles of the alkali salts contained in thefeed material and the total number of moles of zinc and lead. Note thathereinafter this value will be referred to as the “alkalisalt/(zinc+lead) molar ratio”.

The feed material powder is mixed with water to produce a slurry. Inthis case, sometimes the ratio of the powder and water is madesubstantially constant, but for more efficient treatment, it is alsopossible to find the ratio of mixture of the powder and water based onthe mass ratio of the alkali salts and the alkali salt/(zinc+lead) molarratio.

When the content of the alkali salts is high, it is sufficient toincrease the ratio of water. Further, when the content of the alkalisalts is low, it is desirable to produce the slurry by a relatively lowratio of water to an extent enabling the amount of reduction of thealkali salts required to be secured.

When not removing the alkali metals and halogen elements from the feedmaterial powder, the ability of the secondary dust to deposit inside theexhaust gas system of the rotary hearth type reduction furnace 5 risesand the exhaust gas passages will sometimes be clogged.

This is because at a high temperature of 800° C. or more, zinc oxide(sometimes also containing lead) and alkali salts easily react toproduce substances having low melting points and strong depositionabilities. These deposit on the waste heat boiler 8 or the heatexchanger 9 at around 500° C.

For example, when reducing iron oxide powder, with a rotary hearth typereduction furnace, compared with a rotary kiln or other process, thereis less flying iron oxide powder, so the rate of concentration of thealkali salts in the secondary dust is high.

Results of operation for reducing iron oxide as compiled by theinventors show that in general the rate of concentration of alkali saltsin the secondary dust becomes 10 to 200 fold. Therefore, if the ratio ofalkali salts in the feed material powder becomes 0.1 mass %, dependingon the feed material conditions and the operating conditions of therotary hearth type reduction furnace, the content of alkali salts in thesecondary dust till exceed 10 mass %. Further, if the ratio of alkalisalts in the feed material powder becomes 0.2 mass %, the content of thealkali salts in the secondary dust will become 20 to 40 mass %. In bothcases, they pose big problems.

In these cases, the problem of deposition of the secondary dust in theexhaust gas treatment system arises even if strictly limiting theconditions of the temperature or flow rate of the exhaust gas in theexhaust gas treatment facility.

Therefore, when the ratio of the alkali salts in the feed materialpowder is 0.1 mass % or more, in particular 0.2 mass % or more, themethod of reducing the alkali salts of the secondary dust according tothe present invention proves effective.

Further, it is also effective to work the present invention under feedmaterial conditions of an alkali salt/(zinc+lead) molar ratio of 0.1 ormore.

Alkali salts are substances with high deposition abilities around 500°C. even standing alone, but become substances with further higherdeposition abilities when bonded with one or more of zinc oxide, zincchloride, lead oxide, and lead chloride.

The inventors discovered that when mixing alkali salts into zinc andlead oxides up to 10 mol %, the deposition ability does not become thathigh, but if mixing alkali salts in amounts of 10 mol % or more, thedeposition ability of the secondary dust at around 500° C. becomesremarkably high.

Therefore, if the alkali salt/(zinc+lead) molar ratio of the feedmaterial powder is 0.1 or more, even if the mass of the alkali salts inthe feed material powder is 0.1% or less, the problem of deposition ofsecondary dust will arise in some cases, so under such a condition, itis effective to work the method of the present invention.

In the present invention, first, the powder of the feed material ismixed with water inside the agitation tank 1 to form a slurry and makethe water-soluble alkali salts dissolve in the water.

At this time, it is necessary to sufficiently mix the mixture byagitation. Any agitation method may be used, but agitation by impellerrotation, agitation by flow of water, and agitation by gas intake areeffective.

The inventors researched the conditions for stirring a slurry and as aresult discovered that if the ratio of mixture of the powder and wateris suitable, sufficient agitation enables the alkali salts to bedissolved and a slurry containing powder uniformly dispersed to beproduced.

In the case of a powder mainly comprised of iron oxide, manganese oxide,zinc oxide, etc. and also containing about 8 to 15 mass % of carbonpowder with respect to the overall mass, making the mass ratio of thepowder and water 1:1.5 or more is necessary for good mixture byagitation.

In the case of a mass ratio of water less than this, the dynamiccharacteristics of the slurry will become insufficient and dissolutionof the alkali salts in water and uniform mixture of the slurry willbecome impossible. Further, preferably, the mass ratio of the powder andwater should be made 1:1.8 or more. Under these conditions, the mixingof the slurry becomes further sufficient.

The powder-water mass ratio can also be changed by the mass ratio of thealkali salts contained in the feed material powder. When the mass ratioof the alkali salts is extremely high, for example, when she content ofthe alkali salts is 0.4 mass % or more with respect to the total mass ofthe powder or when the alkali salt/(zinc+lead) molar ratio is anextremely high 1:0.2 or more, the powder-water mass ratio is madehigher.

In general, the powder-water mass ratio should be in the range of 1:1.8to 1:5. The reason is that when the ratio is a high one over 1:5, theagitation and mixing states will not be improved any further, while theproblem will arise of the load on the dehydrator becoming larger whendehydrating this slurry in a later step.

However, for example, when an extremely large amount of the alkali saltsis contained such as when the content of the alkali salts becomes 1 to 2mass % or more, the powder-water mass ratio is made 1:5 to 1:15 or so.

It is effective to raise the water temperature for the purpose ofraising the speed of dissolution of the alkali salts and shortening theagitation time. Experiments of the inventors have shown that, comparedwith a water temperature of 20° C., at 30° C., the speed of dissolutionof the alkali salts becomes 1.2 times faster and at a water temperatureof 40° C. greatly increases about 3-fold.

Further, under conditions of a water temperature of 80° C., the speed ofdissolution of the alkali salts becomes about 5 times faster and theeffect is further increased, but with a water temperature of more than80° C., there is almost no further effect of increase of the speed ofdissolution of the alkali salts, the amount of energy consumed inraising the water temperature increases, and there is the detrimentaleffect of the generation of vapor at the slurry agitation tank.

Therefore, the upper limit of the slurry heating temperature forpromoting dissolution of the alkali salts is 80° C., preferably 40° C.to 80° C.

If in this temperature range, 15 to 30 minutes of slurry agitationenables over 95% of the alkali salts to be dissolved.

On the other hand, when the feed material powder contains relativelylarge amounts of zinc and lead, it is necessary to adjust the pH of thewater of the slurry. Zinc and lead are amphoteric metals and dissolve inacidic solutions or high pH strongly alkali solutions.

As a result, they become causes of contamination of water. Further,there is resulting loss of the useful metals of zinc and lead.Therefore, the pH is adjusted so that the zinc and lead will notdissolute in the water.

Experiments by the inventors showed dissolution of zinc and lead underacidic conditions of a pH of 7 or less. In particular, under conditionsof a pH of 5 or less, the zinc and lead dissolved at considerably fastspeeds.

Experiments by the inventors showed that with a slurry of a powder-watermass ratio of 1:2, under conditions of a pH of 3.5 and a watertemperature of 55° C., 35% of the zinc and 23% of the lead dissolvedwith 20 minutes of stirring. As a result, the concentration of heavymetals in the water rose and after-treatment of the water afterdehydration became necessary.

When the pH of the water becomes 7 or more, the zinc and lead will nolonger dissolve much at all. On the other hand, the zinc and lead willstart to dissolve from the point where the pH exceeds 11. This isbecause the zinc and lead-become zinc acid ions and lead acid ions andreact with the alkali ions.

Experiments by the inventors showed that with a slurry of a powder-watermass ratio of 1:2, with 20 minutes of agitation at a water temperatureof 55° C. and a pH in the range of 7 to 11.5, the; rates of dissolutionof zinc and lead were 5% or less.

On the other hand, when the-pH exceeded 11.5, the rate of dissolutionincreased. At a pH of 12, the rate of dissolution became 8.8%.

Therefore, in the case of a feed material powder containing a certainextent or more of, in general 0.3 mass % or more of, zinc and lead, thepM of the water forming the slurry is preferably in the range of 7 to11.5.

After the end of the agitation, the slurry pump 2 is used to send theslurry to the dehydrator 3. The dehydrator 3 separates the water fromthe powder of the slurry. The moisture content of the powder afterdehydration is made not more than 1:0.4 in terms of powder-water massratio (by the general indication of moisture content, corresponding to29 mass %).

The powder-water mass ratio is determined by the conditions enablingefficient reduction treatment of the rotary hearth type reductionfurnace. The detailed reasons will be given later.

If the powder-water mass ratio can be made 1:0.4 or less, any type ofdehydrator 3 can be used, but a high pressure press dehydrator,centrifugal dehydrator, double-roll press type dehydrator, etc. ispreferable.

If high performance machines of these types, it is possible to make thepowder-water mass ratio 1:0.4 or less even with powder comprised ofparticles of an average particle size of 3 to 60 μm. Note thathereinafter, powder containing water after dehydration will be referredto as “dehydrated cake”.

Under the slurry agitation conditions of the present invention, 90 to95% or more of the alkali salts dissolve in the slurry water, so theconcentration of alkali salts in the dehydrated cake is substantiallydetermined by the powder-water mass ratios of the slurry and thedehydrated cake, that is, the concentration of the alkali salts in thewater and the ratio of water remaining in the powder.

For example, when the powder-water mass ratios of the slurry anddehydrated cake are 1:2 and 1:0.3 respectively, the content of alkalisalts of the dehydrated cake becomes the about 15% of the feed materialpowder. That is, about 85% of the alkali salts is removed.

The dehydrated cake is reduced by sintering in the rotary hearth typereduction furnace 5. In general, dehydrated cake cannot be reduced inthe rotary hearth type reduction furnace 5 as it is, so a moldingmachine 4 is used to shape it into masses of diameters or lengths of 25mm or less (shaped articles). These shaped articles are reduced in therotary hearth type reduction furnace 5.

As the types of the shaped articles, there are the spherical pelletsproduced by pan-type granulating machines, briquettes compression moldedby placing the powder in recesses of rolls, cylindrical pellets shapedby extrusion from nozzles, etc.

When using a pan-type granulating machine as the molding machine 4, forgranulation, a moisture content of the feed material powder of 9 to 13mass % is suitable. This moisture content corresponds to a powder-watermass ratio of 1:0.09 to 0.15. In general, it is a value lower than thelower limit of moisture able to be reached by the dehydrator 3.

Therefore, in this case, it is necessary to dry the powder afterdehydration. With a general drying method, the alkali salts will notevaporate, so the content of alkali salts will not change even afterdrying.

The moisture content of the dried powder is reduced to 8 to 13 mass %,then a pan-type granulating machine is used to produce average 10 to 20mm spherical pellets. These spherical pellets have low porosities or 22to 30%. Under high temperature conditions such as the atmospherictemperature of the rotary hearth type reduction furnace, explosions willoccur along with the evaporation of the moisture.

For this reason, in general, a dedicated drier is used to dry thespherical pellets to a moisture content of 1 mass % or less, then thespherical pellets are fed to the rotary hearth type reduction furnace.

Therefore, when using a pan-type granulating machine to shape thepowder, in addition the configuration of equipment shown in FIG. 1, adehydrated cake drier and spherical pellet drier become necessary. Theseare placed between the dehydrator 3 and molding machine 4 and betweenthe molding machine 4 and rotary hearth type reduction furnace 5respectively.

Further, when using a compression molding machine as a molding machineto produce the briquettes, similarly a drier is used to reduce themoisture content to 2 to 15 mass %, then the feed material powder isshaped.

To raise the shaping strength, it is possible to use an organic binder(corn starch etc.) When using this method, it is possible to producebriquettes with relatively low moisture contents. In many cases, thebriquette drying step becomes unnecessary.

In general, when the porosity of the briquettes is less than 35%, underthe high temperature conditions such as the atmospheric temperature ofthe rotary hearth type reduction furnace 5, explosions will occur alongwith the evaporation of the moisture, so it is important that theporosity be made 35% or more.

In this case, even briquettes with a moisture content of 16 to 20 mass %will not explode in the furnace. In this way, when shaping briquettesusing the briquette molding machine, in addition to the configuration ofequipment of FIG. 1, a dehydrated cake drier becomes necessary. This isplaced between the dehydrator 3 and the molding machine 4.

It is possible to produce the shaped articles using any of the above twotypes of molding machines, but using a nozzle extrusion type moldingmachine to produce cylindrical pellets is particularly effective and anImportant means of the present invention.

FIG. 1 shows the layout of a facility in the case of working thisshaping method. Cylindrical pellets have high porosities, so the shapedarticles will not explode even if the speed of evaporation of themoisture is high. Therefore, even if the powder-water mass ratio is1:0.4 or so, the articles can be directly fed into the rotary hearthtype reduction furnace.

In the case of a nozzle extrusion type molding machine, if the feedmaterial moisture content is 16 mass % or less, extrusion of the powderinto the nozzle will become difficult and smooth shaping will not bepossible. Therefore, the range of the moisture content of the powder isimportantly, in terms of the powder-water mass ratio, 1:0.2 to 1:0.4.

The shaped articles have a porosity, indicating the ratio of the spacesbetween the powder particles in the dry state, of 40 to 60% so arestructured to enable water vapor to quickly pass through them.

Therefore, in the present invention, when using a nozzle extrusion typemolding machine, it is possible to omit the drying step of the powder orshaped articles. Therefore, as shown in FIG. 1, preparing the feedmaterial by the combustion of slurry agitation, dehydration, and nozzleextrusion of the feed material powder is extremely economical. Accordingto this method, it is possible to reduce powder containing alkali saltsby a simple configuration of equipment.

Further, the inventors discovered that even if producing shaped articleswith high porosities by methods other than nozzle extrusion, it ispossible to supply the shaped articles to the rotary hearth typereduction furnace without a drying step.

That is, they confirmed that it is possible to produce shaped articleshaving porosities of 40 to 60% in the same way as nozzle extrusion evenby the method of hardening dehydrated cake by compression or anothermethod and then dividing the result or by shaping using a roll typecompactor having stripe patterns.

In particular, if using a high pressure press type dehydrator, it ispossible to produce a plate-shaped dehydrated cake with a porosity of 40to 60% and a thickness of 20 to 50 mm and possible to divide thisdehydrated cake into suitable sizes to produce shaped articles.

When the powder-water mass ratio of the shaped articles is 1:0.2 orless, the binder effect caused by the moisture is not manifested and thearticles easily crumble.

Further, when the powder-water mass ratio is 1:0.4 or more, the shapedarticles become too soft and the problem of deformation or adhesion atthe time of transport surfaces.

Therefore, even when producing the above shaped articles, thepowder-water mass ratio should be 1:0.2 to 0.4. Dote that hereinafter,in the present invention, the shaped articles of cylindrical pelletsproduced by the above method or nozzle extrusion will be referred to as“moist shaped articles”.

The shaped articles produced by the above method (spherical pellets,briquettes, or moist shaped articles) are fed into the rotary hearthtype reduction furnace 5. The rotary hearth type reduction furnace 5 hasa heat zone with the function of heating the shaped articles to causethe water to evaporate and the function of raising the temperature ofthe shaped articles.

The shaped articles raised to a high temperature in this heat zone enterthe further higher temperature reducing atmosphere reduction zone. Inthis reduction zone, the shaped articles are heated to 1100° C. or moreto cause a reduction reaction. Inside the shaped articles, the reduciblemetal oxides and carbon react to produce metal.

The residence time of the shaped articles in the furnace is generally 10to 20 minutes. At this time, carbon monoxide is produced from the shapedarticles. Zinc, lead, and other metals with high vapor pressuresvaporize at this time and are released into the furnace gas of therotary hearth type reduction furnace 5 from the shaped articles alongwith the carbon monoxide.

At this time, since the temperature of the shaped articles is 1000° C.or more, the high vapor pressure alkali salts also evaporate from theshaped articles and are released into the furnace gas.

The size of the moist shaped articles should be a maximum of not morethan 20 to 25 mm or so when shaped close to spheres. The reason is thatif the shaped articles are large, the conduction of heat inside theshaped articles is slow, the time for the evaporation of moisture andreaction is prolonged, and as a result the problem arises that theamount of production of the rotary hearth type reduction furnace isreduced.

Further, in the case of reducing shaped articles of a size of the abovesize or more, the problem arises of a difference in reduction ratesbetween the surface and insides.

The shaped articles are not necessarily spherical, so in general it ispreferable to express their sizes by volume. In the present invention,if expressing the size of the moist shaped articles required for auniform reduction reaction by volume, it should be not more than 10000mm³.

The reduced shaped articles containing the reduced metals (iron, nickel,manganese, etc.) is ejected by a screw ejector from the rotary hearthtype reduction furnace 5 and cooled by the reduced shaped articlecooling device 9 to obtained the finished product.

The reduced shaped articles are used as feed material for producingmolten iron by a steelmaking blast furnace, electric furnace, cupola, orother furnace having a melting function. In particular, in the case ofreduced shaped articles to be charged into a steelmaking blast furnaceor cupola where zinc, lead, an alkali salts become causes obstructingoperation, it is necessary to reduce the contents of these ingredients.

Therefore, a feed material powder with high contents of zinc, lead, andalkali salt is preferably treated using the method of the presentinvention in a rotary hearth type reduction furnace, then used in afurnace having a melting function.

On the other hand, the exhaust gas produced inside the rotary hearthtype reduction furnace 5 and the zinc or alkali salts etc. are exhaustedto the exhaust gas discharge duct 7. This exhaust gas has a hightemperature of around 1000° C., so is cooled by the waste heat boiler 8and heat exchanger 9.

The heat exchanger 9 produces heated air. This heated air is used forthe combustion air to reduce the amount of fuel. FIG. 1 shows theconfiguration of a facility provided with a waste heat boiler and a heatexchanger, but sometimes just one of the two is provided. Further,depending on the plant, sometimes there is no such waste heat recoveryfacility and a sprinkler system is used to cool the exhaust gas bysprinkling it with water.

After the exhaust gas temperature falls to 200° C. or less, the dustcollector 10 recovers the secondary dust. The secondary dust iscomprised of fine particles, so the dust collector 10 may be a bagfilter type or a wet type. The exhaust gas from which dust has finishedbeing collected is released into the atmosphere from the flue 11.

The present invention provides technology for preventing alkali salts orinorganic compounds comprised of zinc, lead, alkali metals, oxygen,and/or halogens from depositing inside the exhaust gas treatment systemof the rotary hearth type reduction furnace 5. It has no waste heatrecovery system and is also effective when using a water sprinklingdevice to form a system to cool the exhaust gas by sprinkling water.

For example, when the content of the zinc or alkali salts in the feedmaterial totals about 3 mass % or more, the problem of deposition ofsecondary dust frequently, occurred even in an exhaust gas treatmentsystem of such a simple configuration.

Further, in the case of an exhaust gas treatment system provided with atleast one of the waste heat boiler 8 and heat exchanger 9, sometimes theexhaust gas passages inside these are only 20 to 50 mm in interval. Themeasure for prevention of deposition of secondary dust according to thepresent invention is particularly effective in this case.

When using dust produced from steelmaking processes or steelmaking dusttreatment furnaces as materials, this secondary dust contains largeamounts of zinc and lead.

In the rotary hearth type reduction furnace 5, the shaped articles areplaced stationarily on the hearth, so compared with a rotary kiln orother process, there is little feed material powder flying off into theexhaust gas. For example, when reducing powder mainly comprised of ironoxide but including a large amount of zinc, there is little flyingiron-oxide powder.

In the experiments conducted by the inventors, the content of iron oxidein the secondary dust was extremely low and no more than severalpercent. That is, in the case of little intermixture of iron oxide inthe secondary dust, the zinc concentration rate of the secondary dust ishigh.

Further, in the method of the present invention, feed material fromwhich part of the alkali salts has been removed is used, so the entry ofalkali salts to the secondary dust is suppressed and as a result, therates of concentration of the zinc and lead become higher.

As a result, when treating the relatively high zinc content blastfurnace dust, converter dust, electric furnace dust, etc., the T. Zn canbecome 50 mass % or higher and sometimes even reach 60% at the maximum.The zinc concentration secondary dust has a high value as a zincrefining material.

In this way, when recovering zinc or lead of a feed material powder on apriority basis, it is important to suitably adjust the reactionconditions of the shaped articles. When the temperature of the exhaustgas of the rotary hearth type reduction furnace 5 is 1000 to 1100° C. orso, the vapor pressure of the zinc and lead is low and even if reduced,the rate of separation by evaporation from the shaped articles is small.

Therefore, for the purpose of promoting the separation by evaporation ofzinc or lead, the temperature of the gas in the reduction zone of therotary hearth type reduction furnace is made relatively high. Results ofanalysis by the inventors revealed that with a temperature of 1200° C.or more, the speed of separation by evaporation of zinc or lead is high.

Further, when the temperature of 1200° C. or more continues for 8minutes or more, it is learned that the dezincification ratio and/ordeleadification ratio becomes 85% or more.

Further, if making the gas temperature 1280° C. or more, it is learnedthat the dezincification ratio and/or deleadification ratio becomes 95%or more.

As explained above, in the present invention, the powder containing thealkali metals and halogen elements is washed and then treated in therotary hearth type reduction furnace, but the method of washing only thepowder containing large amounts of alkali metals and halogen elements inthe material powder in the feed material powder and then mixing theresult with a feed material powder with low contents of alkali metalsand/or halogen elements is also within the scope of the presentinvention.

That is, when mixing iron foundry dust and iron ore together for use asa material, if making the powder with high contents of alkali metals andhalogen elements a slurry and removing the alkali salts, it is possibleto substantially achieve the desired object even without washing theiron ore.

In this way, the using a feed material powder containing large amountsof zinc or lead, the effect of the present invention is large andoperation without any problem in the treatment of exhaust gas becomespossible.

The dust produced from rotary kilns and other primary zinc concentratingplants, steelmaking electric furnaces, steelmaking converters, and blastfurnaces, the sludge produced from the zinc plating process, and othermaterial powder containing lots of zinc and load are treated by a rotaryhearth type reduction furnace to obtain secondary dust which is used asa high priced zinc feed material.

Further, the secondary dust obtained by a dust collector treatingexhaust gas produced by operation by the method of the present inventionis used as a feed material for producing metallic zinc, zinc oxide, andother zinc products in zinc refining plants.

When the zinc contains a relatively large amount of lead, it may betreated directly at the wet electric refining system or zinc refiningmelting furnace to recover the metal zinc without any pretreatment.

EXAMPLE 1

The facility shown in FIG. 1 was used to work the present invention. Theresults are shown as Example 1. In the facility of FIG. 1, thedehydrator 3 is a double-roll press type. Further, the molding machine 4is a nozzle extrusion type. In all operations, the slurry agitation timeat the agitation tank 1 was made 20 minutes.

In the rotary hearth type reduction furnace 5, the reaction temperaturein the reduction zone was made about 1300° C. and the treatment wasperformed for 10 to 15 minutes. The waste heat boiler 8 and the heatexchanger 9 of the exhaust gas treatment system were provided withstrikers and soot blowers as devices for removing deposits.

The temperature of the exhaust gas inside the waste heat boiler 8 was850 to 950° C. at the inlet and 450 to 600° C. at the outlet. Further,the temperature of the exhaust gas at the inside of the heat exchanger 9was 450 to 600° C. at the inlet and 200 to 300° C. at the outlet.

The treatment capacities of the dehydrator 3 and the molding machine 4were both 25 tons/hour (wet amount converted to 25% moisture), while thecapacity of the rotary hearth type reduction furnace 5 was 23 tons/hour(wet amount converted to 25% moisture).

Further, the facility shown in FIG. 2 is used for Example 1 andComparative Examples 1 to 3. This also has a capacity of 23 tons/hour(wet amount converted to 25% moisture). This facility is provided with abriquette type molding machine 4 and a powder drier 12 between thedehydrator 3 and molding machine 4. The powder drier 12 dries thedehydrated cake, while the molding machine 4 shapes the dehydrated cakeand sets the moisture content to a required value.

The waste heat boiler 8 and the heat exchanger 9 of the exhaust gastreatment system shown in FIG. 2 are also provided with strikers andsoot blowers as devices for removing deposits. Note that the results ofoperation for the examples are shown in Table 1, while the results forthe comparative examples are shown in Table 2.

Comparative Example 1 and Example 1 are examples of operation using afeed material mainly comprised of iron ore with relatively low contentsof alkali salts, zinc, and lead. This feed material has a somewhat highratios of alkali salts of 0.21 mass % and an alkali salt/(zinc+lead)molar ratio of a high 1.05.

An example of use of the facility shown in FIG. 2, but treating theshaped articles at 1230° C. for 15 minutes without working the presentinvention is shown in Comparative Example 1. With operation by thistreatment, secondary dust of 8.3 kg/ton of shaped-articles was producedand the concentration of dust in the exhaust gas was about 5 mg/Nm³.

The total of the alkalis and halogens in this secondary dust was a high12.7 mass %. Further, the ratio of the alkalis and halogens with respectto the zinc and lead was also high.

That is, this secondary dust was formed with complex inorganic compoundscomprised of zinc, lead, alkalis, oxygen, and halogens. The inorganiccompounds have melting points of about 420° C. and exhibited extremelyhigh deposition ability at 400 to 600° C. As a result, in ComparativeExample 1, deposition of secondary dust at the heat exchanger 9 could berecognized.

However, in Comparative Example 1, the concentration of dust in theexhaust gas was low, so after about 2 months, the effects of clogging ofthe heat exchanger 9 by deposition of secondary dust appeared.

Note that here, the reason why the alkalis and halogens in the secondarydust are not labeled as “alkali salts”, but labeled as “alkalis” and“halogens” is that the alkalis and halogens in the secondary dust formcomposite inorganic compounds with zinc, lead, etc. and mostly are notin the form of simple alkali salts.

The exact same feed material was used for working the present inventionin Example 1. Since the concentration of alkali salts in the leedmaterial powder was low and almost all of the alkali salts was containedin the blast furnace dust, only the blast furnace dust was mixed withwater.

Note that the mass ratio of the blast furnace dust was 25%. 95% of thetotal alkali salts was contained in the blast furnace dust.

By washing the blast furnace dust, the powder-water mass ratio in theagitation tank was lowered. Note that in the examples, the power-watermass ratio is shown as the “water/powder ratio” as so to facilitateentry into the tables.

In Example 1, the water/powder ratio was 1.56. Further, the watertemperature was also set to a low 35° C. The water/powder ratio at thedehydrator 3 was 0.32. As a result, the content of alkali salts in thedehydrated cake fell and the content of alkali salts in the shapedarticles dropped sharply to 0.05 mass %.

The pH of water is a slightly acidic 6.2, so a small amount ofdissolution of zinc and lead was observed, but the amounts contained inthe feed materials are originally small, so no actual problems arose.

This dehydrated cake was used as a feed material to make shaped articleswhich were then treated in a rotary hearth type reduction furnace 5 at areduction zone gas temperature of 1280° C. for 15 minutes.

The volume of the shaped articles exceeded 10000 mm³, so themetallization rate of iron and the dezincification rate were somewhatlow, but the result was sufficient for use.

In Example 1, the rate of production of secondary dust fell to 6.2kg/ton of shaped articles. Further, the ratio of alkali+halogen fell to5.8 mass %.

The secondary dust had almost no deposition ability. No remarkabledeposition of secondary dust to the heat exchanger 9 could be observed.

Example 2 is an example of operation treating feed material mainlycomprised of converter dust and blast furnace dust. The feed materialhad..a content of alkali salts of an intermediate 0.85 mass %, but ahigh alkali. salt/(zinc+lead) molar ratio of 1.0.

The concentration of alkali salts in the feed material powder was anintermediate one, but the water/powder ratio of the agitation tank 1 wasset to a somewhat low 1.9. The water temperature was made 48° C. toincrease the speed of dissolution of the alkali salts. The water/powderratio at the dehydrator 3 was 0.28.

As a result, the content of alkali salts of the dehydrated cake fellsharply to 0.14 mass %. The dehydated cake was shaped into shapedarticles which were treated in the rotary hearth type reduction furnace5, whereupon the rate of production of secondary dust became 15.7 kg/tonof shaped articles and the concentration of dust in the exhaust gasbecame about 11 mg/Nm³. Further, the (alkali+halogen) ratio was 4.6 mass%.

The secondary dust had almost no deposition ability. No remarkabledeposition of secondary dust to the heat exchanger 9 could be observed.The T. Zn of the-secondary dust recovered by the dust collector 10 was ahigh 51. 5 mass %. Further, the T. Pb was also 10.8 mass %. Thesecondary dust could be used as a good feed material for a zinc refiningmelting furnace for the production of metallic zinc and metallic lead.

On the other hand, Comparative Example 2 is an example of operationreducing a feed material powder the same as Example 2 using the facilityof FIG. 2, but not using the present invention.

The alkali salt content was 0.85 mass %, while the alkalisalt/(zinc+lead) molar ratio was 1.0, so the amount of(alkalis+halogens) in the secondary dust became a high 13.6 mass %.

The melting point of this secondary dust was about 460° C. Thedeposition ability of secondary dust at 450 to 650° C. was extremelyhigh.

The ratio of zinc and lead and the amount of production of secondarydust were greater than with Comparative Example 1. The concentration ofdust in the exhaust gas was about 14 Mg/Nm³. As a result, after abouttwo weeks, the effects of clogging of the heat exchanger 9 due todeposition of secondary dust appeared.

The content of the zinc and lead in the secondary dust was 42.7 mass %or lower compared with Example 2. The value as a feed material for zincand lead was low.

As a result, when using the above secondary dust in a zinc refiningmelting furnace, pre-treatment becomes necessary for remolding thealkalis and halogens before that and therefore the problem arises of anincrease in the costs of zinc refining.

Example 3 is an example of operation for treating a feed material mainlycomprised of electric furnace dust and fine particle scale of steelrolling. In this material, the content of the alkali salts was 0.7 mass%, while the alkali salt/(zinc+lead) molar ratio was 0.21.

The water/powder ratio of the agitation tank was set to 3.9, while thewater temperature was set to 55° C. The water/powder ratio of thedehydrator 3 was 0.38.

As a result, the content of the alkali salts in the dehydrated cake fellgreatly to 0.1 mass %. When shaping this dehydrated cake into shapedarticles and treating them in the rotary hearth type reduction furnace5, the rate of production of secondary dust was a relatively high 62.9kg/ton of shaped articles and the (alkali+halogen) ratio was a low 2.2mass %.

The above secondary dust had almost no deposition ability. No depositionof secondary dust at the heat exchanger 9 could be observed.

The secondary dust recovered by the dust collector 10 contained 55.1mass % of T. Zn and 12.8 mass % of T. Pb. The above secondary dust was agood feed material for a zinc refining melting furnace. Metallic zincand metallic lead could be produced from this secondary dust.

Example 4, Example 5, and Comparative Example 3 are examples ofoperation treating electric-furnace dust and zinc concentration rotarykiln dust as main feed materials. These feed material powders are highin contents of zinc and lead and extremely high in alkali salt content,that is, 3.31 mass %.

Example 4 and Example 5 are examples of operations working the presentinvention. Further, Comparative Example 3 is an example of operation inthe prior art.

Examples 1 to 3 show operations mainly aimed at the production ofreduced iron. The main object of the above operation is the recovery ofsecondary dust concentrated in zinc and lead as a feed material fornonferrous metal refining.

Note that the feed material contained about 0.2% more chlorine than themass ratio of the alkali metals and halogen elements for forming thealkali salts. According to analysis by X-ray diffraction, there was asmall amount of zinc-chloride present in the feed material, so it isbelieved that the above excess chlorine reacted with the zinc.

In Example 4, since the alkali salt content of the feed material powderwas high, the water/powder ratio of the agitation tank 1 was set to 7.5and the water temperature to 60° C. The water/powder ratio at thedehydrated cake fell greatly to 0.1 mass %.

This dehydrated cake was shaped into shaped articles which were treatedat the rotary hearth type reduction furnace 5. The reaction conditionwas a gas temperature in the reduction zone of 1350° C. and a totalresidence time in the furnace of 12 minutes. Note that the residencetime in the gas temperature part of 1200° C. or higher was 9 minutes.

As a result, the dezincification rate of the shaped articles was 95% ormore, that is, almost all of the zinc could be recovered.

The secondary dust of Example 4 had a low (alkali+halogen) ratio of 1.7mass %. As a result, there was almost no deposition ability of thesecondary dust. The rate of production of the secondary dust was anextremely high 241.7 kg/ton of shaped articles, while the concentrationof dust in the exhaust gas was a high one of about 180 mg/Nm³.

Therefore, in the waste heat boiler 8 and the heat exchanger 9, thestriker was frequently made use of. Due to this, no remarkabledeposition of secondary dust could be observed.

The secondary dust recovered by the dust collector 10 contained 64.9mass % of T. Zn and 9.4 mass % of T. Pb and further had small contentsof alkalis and halogens, so was very good as a feed material for zincand lead. This was used as a feed material for electric wet type zincrefining for production of metallic zinc.

Example 5 also uses the present invention, but the pH of the slurrywater in the agitation tank was a too high 11.9, so parts of the zincand lead dissolved in the water. As a result, the T. Zn of the shapedarticles fell to 13.1 mass % and the T. Pb to 2.9 mass %.

The shaped articles were treated in the rotary hearth type reductionfurnace 5. The reaction condition was a gas temperature in the reductionzone of 1320° C. and a total residence time in the furnace of 15minutes. However, the residence time in the gas temperature part of1200° C. or higher was 11 minutes. As a result, almost all zinc in theshaped articles could be recovered.

The rate of production of secondary dust in Example 5 was 216.7 kg/tonof shaped articles, while the (alkali+halogen) ratio was 1.86 mass %. Asa result, despite the high concentration of dust in the exhaust gas, bytaking measures similar to Example 4, there was almost no deposition ofsecondary dust.

The secondary dust recovered by the dust collector 10 contained 58.8mass % of T. Zn and 8.7 masse of T. Pb or somewhat lower compared withExample 4, but was also a very good feed material for zinc and lead.This was used as a feed material for wet type zinc refining forproduction of metal zinc by electric refining..

In Comparative Example 3, the feed material powder was treated in therotary hearth type reduction furnace 5 without lowering the content ofthe alkali salts. The treatment conditions at the rotary hearth typereduction furnace 5 were substantially the same as in Examples 4 and 5.As a result, the total of the weights of the alkalis and halogens in thesecondary dust become 18.6%.

The conditions of the high zinc content also overlap. This secondarydust was extremely high in deposition ability. Further, theconcentration of dust in the exhaust gas was a large one of about 200mg/Nm³. Due in part to this, the heat exchanger 9 became clogged on day4 after the start of treatment.

As a result, continuous operation of the rotary hearth type reductionfurnace 5 was not possible and therefore economical operation was notpossible.

Further, the secondary dust recovered by the duct collector 10 had highcontents of T. Zn of 52.9 mass % and of T. Pb of 3.6 mass %, but alsocontained large amounts of alkali metals and halogen elements, so couldnot be directly used in a zinc refining process.

Due to these reasons, the above secondary dust had to be cleared ofalkalis and halogens by pre-treatment, so the cost of refining zincgreatly rose. Note that to treat this feed material, the facility shownin FIG. 2 was also operated to bypass the waste heat boiler 8 and heatexchanger 9 and cool the exhaust gas by spraying water.

Due to these measures, there are no longer narrow width parts in theexhaust gas passage, so the period of continuous operation was extendedto 10 days, but again continuous operation was not possible andtherefore the costs rose. Further, the problem remained of the inabilityto recover the waste heat. TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Shapingmethod Briquetting Nozzle Nozzle Nozzle Nozzle extrusion extrusionextrusion extrusion Material conditions Type Iron ore, ConverterElectric Electric Electric blast dust, furnace furnace dust, furnacedust, furnace blast dust, fine zinc zinc dust, powder furnace powderconcentration concentration anthracite dust, rolling rotary kiln rotarykiln powder scale, dust, powder dust, powder coke powder coke coke cokeAverage particle 38 μm   18 μm    9 μm    4 μm    4 μm   size T. Fe56%     51%     58%     43%     43%     Carbon/active 0.88    1.14   0.98    1.13    1.13    oxygen molar ratio NaCl ratio 0.07% 0.18% 0.25%1.56% 1.56% KCl ratio 0.11% 0.33% 0.12% 0.45% 0.45% NaF ratio 0.01%0.12% 0.22% 1.01% 1.01% KF ratio 0.00% 0.03% 0.13% 0.12% 0.12% Alkalisalt mass 0.19% 0.66% 0.72% 3.14% 3.14% ratio Zinc ratio (T. 2n) 0.19%0.88% 3.67% 16.6%  16.6%  Lead ratio (T. Pb) 0.02% 0.11% 1.11% 3.3% 3.3%  Alkali 0.96    0.77    0.22    0.22    0.22    salt/(zinc + lead)molar ratio Slurry in agitation tank 1 Water/powder ratio 1.56    1.9   2.9    7.5    7.4    Water temperature 35     48     55     60    62     (° C.) Water pH 6.2    8.8    10.9     9.2    11.9     Dehydratedcake 0.32    0.28    0.36    0.24    0.26    water ratio Shaped articlesNaCl ratio  0.017%  0.029%  0.036%  0.053%  0.060% KCl ratio  0.027% 0.051%  0.017%  0.015%  0.017% NaF ratio  0.002%  0.019%  0.030% 0.032%  0.037% KF ratio  0.000%  0.005%  0.019%  0.005%  0.006% Alkalisalt mass 0.05% 0.10% 0.10% 0.11% 0.12% ratio Zinc ratio 0.18% 0.88%3.60% 16.1%  13.1%  Lead ratio 0.02% 0.11% 1.13% 3.3%  2.9%  Alkali0.25    0.12    0.03    0.01    0.01    salt/(zinc + lead) molar ratioAverage volume  11,800      6,400       4,100       4,000      5,900       (mm³) Reduction conditions Reduction 1280      1320     1260      1350      1320      temperature (° C.) Reduction time 15    15     13     12     15     (minutes) Reduced product Total iron (T. Fe)79%     73%     75%     66%     64%     Iron oxide ratio 64%     84%    81%     88%     83%     Zinc content 0.04% 0.07% 0.17% 0.83% 0.75%Secondary dust T. 2n 22.4%  51.5%  55.1%  64.9%  56.6%  T. Pb 2.9% 10.8%  12.8%  9.4%  8.7%  Na 1.03% 0.76% 0.56% 0.34% 0.38% K 1.88% 0.95%0.31% 0.12% 0.21% Cl 2.44% 2.04% 1.11% 0.63% 0.58% F 0.45% 0.81% 0.22%0.31% 0.39% Alkali + halogen 5.80% 4.55% 2.20% 1.40% 1.56% Generationrate 6.2    15.7     62.9     241.7     216.7     (kg/ton) Secondarydust None None None None None deposition

TABLE 2 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Shaping method BriquettingBriquetting Nozzle extrusion Material conditions Type Iron ore, blastConverter dust, Electric furnace furnace dust, blast furnace dust, zincpowder anthracite dust, powder coke concentration rotary kiln dust,powder coke Average particle size 38 μm 18 μm 4 μm T. Fe   56%   51%  43% Carbon/active oxygen molar 0.88 1.02 1.12 ratio NaCl ratio 0.07%0.18% 1.56% KCl ratio 0.11% 0.33% 0.45% NaF ratio 0.01% 0.12% 1.01% KFratio 0.00% 0.03% 0.12% Alkali salt mass ratio 0.19% 0.66% 3.14% Zincratio (T. 2n) 0.19% 0.88% 16.6% Lead ratio (T. Pb) 0.02% 0.11%  3.3%Alkali salt/(zinc + lead) 0.96 0.77 0.22 molar ratio Shaped articlesNaCl ratio 0.07% 0.028%  1.56% KCl ratio 0.11% 0.11% 0.45% NaF ratio0.01% 0.33% 1.01% KF ratio 0.00% 0.12% 0.12% Alkali salt mass ratio0.19% 0.03% 3.14% Zinc ratio 0.19% 0.66% 21.6% Lead ratio 0.02% 0.11% 4.3% Alkali salt/(zinc + lead) 0.96 0.77 0.17 molar ratio Averagevolume (mm²) 10,900     6,700    6,000    Reduction conditions Reductiontemperature (° C.) 1280     1320     1320     Reduction time (minutes)15    10    15    Reduced product Total iron (T. Fe)   79%   73%   62%Iron oxide ratio   69%   80%   80% Zinc content 0.04% 0.08% 0.66%Secondary dust T. 2n 19.4% 42.7% 52.9% T. Pb  1.9%  8.2%  8.6% Na 2.22%2.12% 4.43% K 3.81% 3.85% 1.25% Cl 5.66% 7.39% 9.25% F 0.98% 0.26% 3.04%Alkali + halogen 12.7% 13.6% 18.0% Generation rate (kg/ton) 8.3  19.4 307.5   Secondary dust deposition None None None

(II) Regarding Invention For Reducing Steelmaking Waste

FIG. 3 is a view of an embodiment of the present invention for reducingsteelmaking waste.

In FIG. 3, steelmaking waste X, water W, a pH adjuster Y, and acarbon-bearing material C are mixed by agitation in an agitation tank 1,the mixture is concentrated to a slurry in a concentration tank 12, thenthe slurry is sent by a slurry pump 13 to a double-roll press typedehydrator 14. In this dehydration step, the volatile organic substancesin the waste (sodium chloride, potassium chloride, etc.) are removed.

The dehydrated cake dehydrated by the double-roll press type dehydrator14 is sent by a dehydrated cake conveyor 15 to an extrusion type moldingmachine 16 where it is shaped into cylindrical shaped articles. Theseare conveyed by a shaped article conveyor 17 and supplied through ashaped article charging system la to a moving hearth furnace (forexample, a rotary hearth furnace) 19.

The shaped articles reduced by heating in the moving hearth furnace 19become reduced iron F. The gas produced at this time is cooled by a gascooler 20, then the dust is recovered as high zinc secondary dust Dcontaining zinc oxide (ZnO) by a dust collector 21. The exhaust gas isreleased through a blower 22 from the flue 23.

Further, the moisture discharged from the concentration tank 12 anddouble-roll press type dehydrator 14 is collected at a return water tank24 and treated at a water treatment system 25. The water required foragitation of the material is then returned as return water W1 by areturn water pump 26 to the agitation tank 1, while the remainder isdischarged (W2 in the figure).

In the embodiment shown in FIG. 1, the carbon-bearing material C ismixed by agitation in the agitation tank 1, but as shown in FIG. 4, itis also possible to mix by agitation the steelmaking waste W and pHadjuster Y in water, then concentrate the mixture to obtain a dehydratedslurry, add the carbon-bearing material C to this, then knead themixture using a kneader 27.

Further, if the molding machine 16 is one also provided with a kneadingfunction, the kneader 27 may be omitted.

EXAMPLE 2

The present invention was used to treat electric furnace dust(steelmaking waste). The results are shown below.

The present invention was used to wash the electric furnace dust andthen analyze the ingredients. The results are shown in Table 3. Further,the rates of D removal of the different ingredients are shown in Table4.

The test conditions were made a washing solution temperature of 60° C.,a washing water/dust ratio of 10, and an agitation time of 30 minutes.TABLE 3 NaOH added pH Zn Pb Na K Cl TFe No washing 18.9% 1.71% 1.90%1.71% 5.56% 28.8% After 0.0% 6.7 20.0% 1.82% 0.26% 0.21% 2.86% 30.8%washing 9.5% 9.3 21.4% 1.94% 0.33% 0.10% 0.61% 32.6% 10.0% 11.4 21.5%1.94% 0.52% 0.09% 0.68% 32.7% 12.0% 13.0 21.8% 0.92% 0.29% 0.08% 0.64%33.2%

TABLE 4 NaOH added pH 2n Pb Na K Cl After 0.0% 6.7 1.141% 0.56% 87.2%88.5% 51.9% washing 9.5% 9.3 0.002% 0.14% 84.8% 94.9% 90.4% 10.0% 11.40.004% 0.09% 75.7% 95.4% 89.3% 12.0% 13.0 0.464% 53.41% 86.7% 95.8%90.1%

As shown in Table 3 and Table 4, if washing the electric furnace dustwithout adjusting the pH, a weak acidity of pH 6.7 is exhibited and theCl removal ratio at that time is only about 52%, but if adding NaOH andmaking the pH about 9 to 12, the Cl removal ratio rises to about 90% andthe loss of Zn also becomes very small.

Note that even if the pH rose to about 13, the rise in the Cl removalratio leveled off and the Zn loss increased.

The filtrate concentration (ppm) at this time is shown in Table 5. TABLE5 NaOH added pH Zn Pb Na K Cl After 0.0% 6.7 539 33.10 2,550 2,870 8,610washing 9.5% 3.3 0.76 8.76 7,180 2,920 13,500 10.0% 11.4 1.74 5.86 7,4802,880 14,310 12.0% 13.0 221 3,354 12,140 3,040 14,620

In each case, the general discharge standard in Japan, that is, Pb<0.1ppm, is exceeded, so water treatment is required for discharge.

The water treatment, that is, the generally practiced addition of a pHadjuster FeCl₂ and polymer flocculating agent, resulted in Pb<0.1 ppmand enabled discharge.

Next, powder coke was added to the steelmaking waste which was thenheated and reduced. The secondary dust was trapped and subjected to areduction test. The results are shown in Table 6. TABLE 6 (Unit: ppm)NaOH added pH 2n Pb Na K Cl No 55.0% 5.0% 4.2% 4.5% 16.4% washing After0.0% 6.7 65.7% 6.0% 0.7% 0.6% 9.5% washing 9.5% 9.3 70.4% 6.4% 0.8% 0.3%2.0%

The secondary dust when reducing unwashed dust contained about 16% Cl,about 9% (Na+K), and about 55% Zn, so was low in purity of Zn, while thesecondary dust when reducing dust-washed at a pH of about 9 containedabout 2% Cl, about 1% (Na+K), and about 70% Zn (about 68% ZnO), so wasgreatly improved in Zn purity.

Further, the inventors conducted a test using refuse melting furnace flyash instead of the above NaOH as the pH adjuster. The ingredients of therefuse melting furnace fly ash used for the test are shown in Table 7,the results of the test in Table 8, and the removal ratios of thedifferent ingredients in Table 9.

The test conditions were a washing temperature of 60° C., a washingwater/dust ratio of 10, and an agitation time of 30 minutes. TABLE 7 ZnPb Na K Cl TFe Ca Si pH CaO/SiO₂ 6.12% 1.15% 5.15% 4.27% 19.4% 0.7%19.4% 4.23% 11.0 3.0

TABLE 8 Fly ash added pH Zn Pb Na K Cl TFe Ca Si CaO/SiO₂ No washing 018.9% 1.71% 1.90% 1.71% 5.56% 28.8% 1.71% 1.87% 0.60 After washing 10%9.9 19.7% 1.84% 0.37% 0.11% 0.73% 29.2% 3.32% 2.32% 0.94

TABLE 9 Fly ash added pH 2n Pb Na K Cl After 10% 10 0.01% 0.1% 84.9%95.0% 90.4% cleaning

As shown in Table 7, the refuse melting furnace fly ash had a pH ofabout 11 and a strong alkaline nature. This was because the calciumhydroxide (Ca(OH)₂) blown into the exhaust gas of the melting furnace inorder to neutralize the HCl contained in the exhaust gas remains in thefly ash.

Therefore, the fly ash also had a high Ca content and a basicity(CaO/SiO₂) of about 3.

As shown in Table 8, if adding refuse melting furnace fly ash to theelectric furnace dust at a mass ratio of 10% and making the pH about 10at the time of washing, it is possible to raise the Cl removal ratio toabout 90%. The loss of Zn also becomes extremely small.

Further, the basicity (CaO/SiO₂) also rose from the about 0.6 beforeaddition of the refuse melting furnace fly ash to 0.9 or more.

Next, the inventors added powder coke to the electric furnace dust,reduced the dust, and trapped the secondary dust. The results are shownin Table 10. TABLE 10 Fly ash added pH Zn Pb Na K Cl No 0 55.0% 5.0%4.2% 4.5% 16.4% washing After 10% 9.9 69.1% 6.4% 1.3% 0.4% 2.6% cleaning

The secondary dust when reducing unwashed dust contained about 16% Cl,about 9% (Na+K), and about 55% Zn (about 68% ZnO), so was low in purityof Zn, but if adding refuse melting furnace. fly ash to the electricfurnace dust by a mass ratio of 10% and making the pH about 10 at thetime of washing, the dust contained about 2.6% Cl, about 1.7% (Na+K),and about 69%; Zn (about 86% ZnO), so was greatly improved in Zn purity.

INDUSTRIAL APPLICABILITY

According to the present invention, even if using a feed materialcontaining large amounts of alkali metals and halogen elements inoperating a rotary hearth type reduction furnace, it is possible toavoid the problem of deposition of dust at the exhaust gas treatmentsystem and possible to economically reduce metal oxides to produce iron,nickel, and other metal materials.

In particular, the present invention is effective when the exhaust gastreatment system is provided with a waste heat boiler, heat exchanger,or other waste heat recovery system.

Further, by working the present invention, it is possible to raise thepurity of the zinc or lead in the exhaust gas dust and possible torecover the dust as a good zinc and lead resource.

Further, according to the present invention, by washing the steelmakingwaste adjusted in pH, there is the effect that it is possible to providea method of treatment and system for treatment of steelmaking waste ableto separately recover volatile harmful substances (potassium chloride,sodium chloride, etc.) and zinc oxide (ZnO) and further not requiringdrying before charging the feed material into the moving hearth furnaceand, on top of this, the purity of the zinc oxide able to be recoveredis remarkably improved.

By using fly ash of a refuse melting furnace or incinerator furnace forthe pH adjuster, it is possible to eliminate or reduce the amount of useof the expensive NaOH and other chemicals.

Further, since the basicity after washing (CaO/SiO₂) becomes 0.9 ormore, if reducing the washed feed material by a moving hearth reductionfurnace and recycling it as a resource of iron for an electric furnaceetc., it is possible to reduce the amount of CaO used for adjusting thebasicity.

1. A method of reduction treatment of metal oxides characterized byusing as a feed material a powder containing metal oxides and containingalkali metals and halogen elements, mixing said feed material with waterto produce a slurry, then dehydrating this and charging the dehydratedmaterial into a rotary hearth type reduction furnace for reduction.
 2. Amethod of reduction treatment of metal oxides characterized by using asa feed material a powder containing metal oxides and containing alkalimetals and halogen elements, mixing said feed material with water toproduce a slurry, then dehydrating this, mixing the dehydrated materialwith another feed material, and charging said mixture into a rotaryhearth type reduction furnace for reduction.
 3. A method of reductiontreatment of metal oxides characterized by using as a feed material amixed powder of a powder containing metal oxides and containing alkalimetals and halogen elements and a powder containing carbon, mixing saidfeed material with water to produce a slurry, then dehydrating this, andcharging said dehydrated material into a rotary hearth type reductionfurnace for reduction.
 4. A method of reduction treatment of metaloxides characterized by using as a feed material a mixed powder of apowder containing metal oxides and containing alkali metals and halogenelements and a powder containing carbon, mixing said feed material withwater to produce a slurry, then dehydrating this, mixing the dehydratedmaterial with another feed material, and charging said mixture into arotary hearth type reduction furnace for reduction.
 5. A method ofreduction treatment of metal oxides as set forth in any one of claims 1to 4, characterized in that said powder contains a total of at least 0.1mass % of alkali metals and halogen elements.
 6. A method of reductiontreatment of metal oxides as set forth in any one of claims 1 to 5,characterized in that a mass ratio of powder and water in said slurry isat least 1:1.5 and a mass ratio of powder and water in said dehydratedmaterial is not more than 1:0.4.
 7. A method of reduction treatment ofmetal oxides as set forth in any one of claims 1 to 6, characterized byheating and agitating the slurry at 80° C. or less in the production ofsaid slurry.
 8. A method of reduction treatment of metal oxides as setforth in any one of claims 1, 2, and 5 to 7, characterized by using assaid feed material a powder containing both iron oxide and zinc oxideand/or lead oxide and containing alkali metals and halogen elements in aratio alkali/(zinc+lead) between a total of the number of moles ofalkali salts and a total of the number of moles of lead of at least 0.1.9. A method of reduction treatment of metal oxides as set forth in anyone of claims 3, 4, and 5 to 7, characterized by using as said feedmaterial a powder comprised of a mixture of a powder containing bothiron oxide and zinc oxide and/or lead oxide and a powder containingcarbon and containing alkali metals and halogen elements in a ratioalkali/(zinc+lead) between a total of the number of moles of alkalisalts and a total of the number of moles of lead of at least 0.1.
 10. Amethod of reduction treatment of metal oxides as set forth in claim 8,characterized in that a pH of a slurry produced by mixing said powderwith water is 7 to 11.5.
 11. A method of reduction treatment of metaloxides as set forth in claim 9, characterized in that a pH of a slurryproduced by mixing said mixed powder with water is 7 to 11.5.
 12. Amethod of reduction treatment of metal oxides as set forth in any one ofclaims 1 to 11, characterized by shaping said dehydrated material intomoist shaped articles having a porosity of at least 35% and chargingsaid shaped articles into a rotary hearth type reduction furnace forreduction without drying.
 13. A method of reduction treatment of metaloxides as set forth in claim 12, characterized by making a mass ratio ofpowder and water in said dehydrated material 1:0.2 to 1:0.4 and shapingsaid dehydrated material into moist shaped articles having an averagevolume of not more than 10000 mm³.
 14. A method of reduction treatmentof metal oxides as set forth in claim 13, characterized by making amolar ratio of oxygen and carbon contained-in said shaped articles 1:0.6to 1:1.5, charging said shaped articles into a rotary hearth typereduction furnace, and reducing them by leaving them for at least 8minutes at the part of the furnace having a gas temperature or 1200° C.or more.
 15. A method of reduction treatment of metal oxides as setforth in any one of claims 1 to 14, characterized in that said rotaryhearth type reduction furnace is provided with an exhaust gas treatmentfacility having at least one of a waste heat boiler and an airpreheater.
 16. A method of reduction treatment of metal oxides as setforth in any one of claims 1 to 15, characterized in that said powder issteelmaking waste.
 17. A method of concentrating and recovering zincand/or lead characterized by recovering dust in exhaust gas produced inthe method of reduction treatment of metal oxides described in any ofclaims 1 to 16 as feed material for zinc and/or lead.
 18. A method ofreduction treatment of steelmaking waste characterized by: mixing byagitation steelmaking waste, a pH adjuster, and a carbon-bearingmaterial in water, then concentrating the mixture to produce a slurry,pressing said slurry to dehydrate it, extruding said dehydrated materialto shape it into shaped articles, charging said shaped articles into amoving hearth type reduction furnace for reduction and recovering thesecondary dust containing zinc oxide produced.
 19. A method of reductiontreatment of steelmaking waste characterized by: stirring and mixingsteelmaking waste and a pH adjuster in water, then concentrating themixture to produce a slurry, pressing said slurry to dehydrate it,adding and kneading a carbon-bearing material into said dehydratedmaterial, extruding said dehydrated material to shape it into shapedarticles, charging said shaped articles into a moving hearth typereduction furnace for reduction and recovering the secondary dustcontaining zinc oxide produced.
 20. A method of reduction treatment ofsteelmaking waste as set forth in claim 18 or 19 characterized in thatsaid pH adjuster is a substance containing OH— groups.
 21. A method ofreduction treatment of steelmaking waste as set forth in any one ofclaims 18 to 20 characterized in that said pH adjuster is fly ashdischarged from a refuse melting furnace or incinerator furnace.
 22. Amethod of reduction treatment of steelmaking waste as set forth in anyone of claims 18 to 21 characterized in that a pH of the slurry adjustedin pH by said pH adjuster is at least
 8. 23. A method of reductiontreatment of steelmaking waste as set forth in any one of claims 18 to22 characterized in that said dehydrated material contains moisture inan amount of 16 to 27 mass % of said dehydrated material.
 24. A systemfor reduction treatment of steelmaking waste characterized by beingprovided with: an agitation tank for mixing by agitation steelmakingwaste, a pH adjuster, and a carbon-bearing material in water, aconcentration tank for concentrating the agitated mixture to produce aslurry, a dehydrator for pressing the slurry poured on endlessly movingfilter cloth by at least one pair of rolls arranged above and below thecloth so as to dehydrate it, a molding machine for extruding saiddehydrated material from a die to shape it, a moving hearth typereduction furnace for reducing said shaped articles, and a dustcollector for recovering the secondary dust containing zinc oxideproduced in said moving hearth type reduction furnace.
 25. A system forreduction treatment of steelmaking waste characterized by being providedwith: an agitation tank for mixing by agitation steelmaking waste and apH adjuster in water, a concentration tank for concentrating theagitated mixture to produce a slurry, a dehydrator for pressing theslurry poured on endlessly moving filter cloth by at least one pair ofrolls arranged above and below the cloth so as to dehydrate it, akneader for adding and kneading a carbon-bearing material to saiddehydrated material, a molding machine for extruding said dehydratedmaterial from a dies to shape it, a moving hearth type reduction furnacefor reducing said shaped articles, and a dust collector for recoveringthe secondary dust containing zinc oxide produced in said moving hearthtype reduction furnace.